Mounting for piezo-electric devices



NW. 25, 1930, R. 5 0m ,7 3, 3

I MOUNTING FOR PIEZO ELECTRIC DEVICES Filed Sept. 23. 1926 INVENTOR. 1 S. Oil/l C ATTORNEY Patented Nov. 25, 1930 unrrs STATES PATENT OFFICE RUSSELL S. OHL, OF NEW YCRK, N. Y., ASSIGNOR TO AMERICAN TELEPHONE AND TELE GRAPE CGIiPANY, A CORPORATION OF NEW YORK MOUNTING FOR PIEZO-ELEOTRIC DEVICES Application filed September 23, 1926. Serial No. 137,377.

effects on the free vibratory period of the piezo-electric member caused by variations in the surrounding thermal conditions.

A piezoelectric device comprises a crystal of piezo-electric character which may be set into vibration between two conductive electrodes. When such a piezoelectric crystal is placed between the two conductive electrodes in such a way that a gap exists between one or both of the surfaces of the piezoelectric crystal and the corresponding conductive electrodes, so that the piezo-electric crystal may freely vibrate therebetwcen, the vibratory period of the piezo-electric crystal may be varied by changing the size of the gap between the piezoelectric crystal and the corresponding conductive electrodes. In experimenting with such a crystal, it has been found that as the size of the gap changed, the frequency of vibration of the piezoelectric crystal also changed within reasonable limits.

Heretofore, such a piezo-electric crystal was mounted between two metallic conductive plates or electrodes in some suitable mounting in such a way that the gap above mentioned was provided to permit the free vibration of the piezoelectric crystal. The size of the gap was found to be dependent upon the rigidity of some insulating material such as hard rubber, bakelite, etc., used for the base of the mounting. Moreover, in the mountings heretofore used, the external thermal variations brought about expansions and contractions ofthe elements of the mounting, causing uncontrollable changes in the size of the gap existing between the piezo-electric crystal and its corresponding conductive plates orelectrodes. Dueto such changes in the external thermal conditions, the piezoelectric crystal continually assumed a different vibratory period.

One of the principal objects of this invention is theprovis'ion of means in some form of a mounting for a piezo-electric device wherein the undesirable effects of temperature variation on the free vibratory period of the piezoelectric crystal contained therein are avoided or substantially reduced.

lVhile this invention will be pointed out with particularity in the appended claims, the invention, itself, both as to its further objects and features, will be better understood from the detailed description hereinafter following, when read in connection with the accompanying drawing, showing a cr0ss-sectional view of a mounting for a piezoelectric device and the enclosed piezo-electric crystal, which drawing is shown herein merely for the purpose of illustration.

Referring to the drawing, two bases, the upper and lower bases are designated by the reference characters B and B respectively. These bases may be of any material whatsoever, but these are preferably of some insulating material such as hard rubber, bakelite, porcelain, glass, etc. Cemented or otherwise suitably fastened to the bases B and B are two electrodes l? and P respectively, these electrodes forming the means for the transmission or reception of electric current to or from a piczo-electric crystal PE mounted between these electrodes. It will be understood that these electrodes P; and P may be of any material whatsoever but that they are preferably of some conductive material such as brass, steel, nickel, etc. The device of piezo-electric character PE is so placed between theconductive electrodes P and P that a gap of, for example, air, exists between one or both of the surfaces of the piezo-electric device PE and the adjacent conductive electrodes P and P This gap permits the free vibration of the piczo-electric device at a characteristic frequency. Yet, by changing the size of this gap, the vibratory period of the piezoelectric device PE may be effectively changed.

In order to separate the electrodes P and P from each other by some predetermined distance, a tube or cylinder G is employed, this tube or cylinder being preferably coaxial with the piezo-electrie device PE and also coaxial with the adjacent conductive electrodes P and P This tube or cylinder G may be of any material whatsoever, but it is preferably of some such insulating matetially negligible trodes P and P and the linear coeh'icients of thermal expansion of the tube or cylinder G and the conductive electrodes P and P are so' chosen, that any change in the longitudinal dimensions of the conductive electrodes P and P due to any variation in the external temperature conditions, will be equal and opposite to the change taking place in the. longitudinal dimensions of the tube or cylinderG In other words, a, change in the longitudinal dimensions of the conductive electrodes P and? due to an external temperature variation, will be substantially equal. and oppositeto the change in the longitudinal dimension of the tube or cylinder G'due to the same external; temperature variation. .Assuming that al and (Z represent,

respectively, the efiective longitudinal dimensions of the conductive electrodes P and P res ectively, and that D represents the longitudinal dimension of the tube or cylinder G, and furthermore, assuming that K and K represent, respectively, the linear coeificients of thermal expansion of'the conductive electrodes? and P and of the tube or cylinder G, then the relationship above mentioned may be expressed algebraically as follows that Kl=nx1n As the external temperature rises, the distance between the conductive electrodes P and P; will decrease, and vice versa. Furthermore, as the external temperature rises, the longitudinal dimension of the tube or c linder G will increase, and vice versa. T he neralphysical proportions of the tube or cylinder G and conductive electrodes P, and l -are so chosen that the products of any changes in their longitudinal dimensions and their corresponding linear coellicients of thermal expansion will be substantially equal. Yet, it will be understood that these products may be varied so as to bear some fixed relationship to each other to bring about any desired change'in the size of the existing between the piezo-electric device PE and the adjacent conductive electrodes P and P when a change takes place in the external thermal conditions, all of which is within the scope of thisinvention.

While this invention has been shown and described in one particular embodiment merely for thepurpose of illustration, it will said device and one of said electrodes,

be understood thatthe general principles of this invention may be embodied in other and widely varied organizations without departing from the spirit of the invention and the scope of the appended claims.

What is claimed is: r

l. A mounting for a pieZo-electric device which shall be substantially free from external thermal variations, comprising two metallic electrodes disposed so that the piezoelectric device may freely vibrate therebetween and so that there will be a longitudinal movement of the faces of said electrodes due to external thermal variations, and a tube surrounding the piezo-electric device and coaxial therewith maintaining the metallic elecdes a fixed distance apart, said tube prolo-ngitudinal movement due to, said external thermal variations equal to that of the faces of said metallic electrodes.

2. Piezoelectric apparatus comprisingtwo metallic electrodes affected by temperature, a. piezo-electric device located between said electrodes, an air-gap being provided between and, a the piezoelectric decyli'nder coaxial with vice and separating the metallic electrodes by a definite d1stance,sa1d cylinder bemg so pro,-

portioned that any external thermal; varia tion may produce a linear movement of the faces of the metallic electrodes substantially equal and opposite to the linear movement of the cylinder separating the faces of the metallic electrodes.

3; Piezo-electric apparatus comprising two metallic electrodes, a piezo-electric device so placedtherebetween that a gap is provided to permit the free vibration of the pieZo-elec- V tric device, cylinder coaxial with the piezoelectric device separating the metallic electrodes by a predetermineddistance during normal external conditions, the product of the sum of the projecting lengths of said metallic electrodes and their coefficient of thermal expansion bearing a fixed relationship to the corresponding product of the length of the cylinder and its coeficientof thermal ex pansion. i

4. P1Z0-Ql8tfl0 apparatus comprising a pair of bases of insulating material, a pair of metallic electrodes permanently fixed, respectively, to said bases, a piezoelectric device placed between said metallic electrodes, said metallic electrodes being spaced sothat a gap exists between the pieZo-elcctric device and the surface of one of the adjacent metallic electrodes so thatthe piezo-electric device may assume a characteristic frequency and freely vibrate at that; characteristic frequency, and a tube of refractory-material surrounding the metallic electrodes and the piezoelectric device and separating the said metallic electrodes by some predetermined distance, the. tube and; the metallic electrodes being so chosen and so'proportioned that said predetermined distance between said 1netallic electrodes will remain constant during any change in the external thermal conditions.

5. The combination of a pair of metallic electrodes, and a hollow cylinder so arranged as to maintain the metal ic electrodes a predetermined distance apart, said cylinder being so proportioned that the effective distance between said metallic electrodes will be substantially independent of temperature variations.

6. The combination of a pair of metallic electrodes which are affected by temperature variations and dielectric means having a predetermined coefficient of thermal expansion for maintaining said metallic electrodes a predetermined distance apart and so that the spacing of said electrodes will be substantially nnafi'ected by said temperature variations.

T. A mounting for a piezoelectric device comprising a pair of electrodes spaced a predetermined distance apart so that a gap exists when the piezo-electric device is placed therebetween, both of said electrodes chang ing in their lengths with temperature variations and tending to correspondingly change the size of said gap, and means of dielectric character having a predetermined coetlicient of thermal expansion for maintaining said distance independent of variations in the condition of the surrounding medium.

8. Piezo-electric apparatus comprising two metallic electrodes a piezo-electric device located between said electrodes, a gap being provided due to the spacing of the electrodes which is greater than the thickness of said piezo-electric device so that the piezo-electric device may vibrate freely therebetween, a cylinder of dielectric material coaxial with the piezo-electric crevice separating the metallic electrodes by a predetermined distance, the product of the sum i .of the projecting lengths of the metallic electrodes and their coefficient of thermal expansion equalling the corresponding product of the length of said cylinder and its coeiiicient of thermal expansion.

9. Piezo-electric apparatus comprising a pair of metallic electrodes both ailected by temperature. a piezo-elcctric device located between said electrodes, said metallic electrodes being placed so that a gap exists between said metallic electrodes and the piez r electric device permitting the free vibratioi' of the piezo-electric device, a cylinder coaiiia l with the piezoelectric device and separating said metallic electrodes by a fixed predetermined distance, said cylinder being of such a longitudinal dime sion and of such a mate rial that any change in the thermal conditions tending to produce a change in its longitudinal dimension will be substantially balanced by a corresponding change in the September, 1926.

RUSSELL S. OHL. 

